Cardiovascular regulation in the period preceding vasovagal syncope in conscious humans

To study cardiovascular control in the period leading to vasovagal syncope we monitored beat‐to‐beat blood pressure, heart rate (HR) and forearm blood flow in 14 patients with posturally related syncope, from supine through to tilt‐induced pre‐syncope. Signals of arterial blood pressure (BP) from a Finapres photoplethysmograph and an electrocardiograph (ECG) were fed into a NeuroScope system for continuous analysis. Non‐invasive indices of cardiac vagal tone (CVT) and cardiac sensitivity to baroreflex (CSB) were derived on a beat‐to‐beat basis from these data. Brachial vascular resistance (VR) was assessed intermittently from brachial blood flow velocity (Doppler ultrasound) divided by mean arterial pressure (MAP). Patients underwent a progressive orthostatic stress test, which continued to pre‐syncope and consisted of 20 min head‐up tilt (HUT) at 60 deg, 10 min combined HUT and lower body suction (LBNP) at −20 mmHg followed by LBNP at −40 mmHg. Pre‐syncope was defined as a fall in BP to below 80 mmHg systolic accompanied by symptoms. Baseline supine values were: MAP (means ±s.e.m.) 84.9 ± 3.2 mmHg; HR, 63.9 ± 3.2 beats min−1; CVT, 10.8 ± 2.6 (arbitrary units) and CSB, 8.2 ± 1.6 ms mmHg−1. HUT alone provoked pre‐syncope in 30 % of the patients whilst the remaining 70 % required LBNP. The cardiovascular responses leading to pre‐syncope can be described in four phases. Phase 1, full compensation: where VR increased by 70.9 ± 0.9 %, MAP was 89.2 ± 3.8 mmHg and HR was 74.8 ± 3.2 beats min−1 but CVT decreased to 3.5 ± 0.5 units and CSB to 2.7 ± 0.4 ms mmHg−1. Phase 2, tachycardia: a progressive increase in heart rate peaking at 104.2 ± 5.1 beats min−1. Phase 3, instability: characterised by oscillations in BP and also often in HR; CVT and CSB also decreased to their lowest levels. Phase 4, pre‐syncope: characterised by sudden decreases in arterial blood pressure and heart rate associated with intensification of the symptoms of pre‐syncope. This study has given a clearer picture of the cardiovascular events leading up to pre‐syncope. However, the mechanisms behind what causes a fully compensated system suddenly to become unstable remain unknown.

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